Method for the removal of impurities from metallic zinc

ABSTRACT

IMPURITIES SUCH AS IROM, COBALT, NICKEL AND/OR MANGANESS ARE REMOVED FROM MOLTEN ZINC BY BINDING THE IMPURITIES WITH SILICON FOLLOWED BY SEPARATION.

United States Patent Ofiice 3,585,985 Patented Aug. 22, 1972 U.S. CI. 7586 8 Claims ABSTRACT OF THE DISCLOSURE Impurities such as iron, cobalt, nickel and/or manganese are removed from molten zinc by binding the impurities with silicon followed by separation.

BACKGROUND Among the elements which are readily picked up by zinc and zinc alloys in the production and utilization of the metal (as in the hot-dip galvanizing of iron parts, for example) are iron, cobalt, nickel and manganese. From the Fe-Zn, Co-Zn, Ni-Zn and Mn-Zn phase diagrams, which resemble one another closely, it is apparent that zinc forms intermetallic, and primarily peritectically melting, compounds with these metals. They materially impair the processing and utilization properties of zinc. Since the solubility in zinc of these elements is very low and the intermetallic compounds are very rich in zinc (for example, one part of the elements Fe, Co, Ni or Mn will bind some 10 to parts of zinc) these compounds manifest themselves even in the structure of zinc and zinc alloys that are very poor in these elements.

In order to avoid the segregation of primary, coarsely crystalline intermetallic compounds in the solidification of zinc and zinc alloys, it is usually specifiedin DIN 1706, Zinc, for examplethat the elements Fe, Co, Ni and Mn shall be limited to a few hundreds or thousandths of one weight percent.

It is a known and common practice to purify zinc and zinc alloys of said impurities, and particularly of Fe, by liquation and distillation. In liquation, zinc and zinc alloys can be purified down to the eutectic concentrations (for example, about 0.018% Fe in the -Fe-Zn system) by letting the primary crystals settle out at temperatures just above the melting point of zinc, for example. However, zinc losses in the separated layer enriched in impurities, the so-called hard zinc, are relatively high.

High-purity zinc may be obtained by distilling Fe-, Co-, Nior Mn-containing zinc or zinc alloys. While the yield is good in view of the thermal decomposition of said intermetallic zinc compounds, power consumption is high and elaborate equipment is needed.

SUMMARY It has now been found that intermetallic compounds of zinc with Fe, Co, Ni and/ or Mn can be converted with silicon to stable silicon compounds (silicides), the very low solubility of silicon in zinc and high-zinc alloys being a factor that greatly favors refining. The zinc purified of Fe, Co, Ni and Mn contains no demonstrable amounts of silicon.

DESCRIPTION The silicon compounds forming with said impurities are crystalline up to the boiling point of zinc, have low specific gravity and are diflicultly wettable by zinc. Thus they can be readily separated.

Since silicon is only very slightly soluble in zinc over the temperature range between the melting point and the boiling point of zinc, the zinc impurities Fe, Co, Ni and Mn can actually be bound only at the interface between zinc melt and silicon crystal. For this reason, provision must be made for adequate contact between the silicon and the melt, for example, by mechanical or inductive stirring, by blowing in the silicon with an inert gas, by dipping, whirling, shaking or pouring the silicon in, or by similar known methods. Moreover, oxidizing atmospheres should be avoided.

Since the binding of the zinc impurities occurs on the solid silicon and consequently is diffusion-dependent to a high degree, it will be advisable to use as high a reaction temperature as possible. The decomposition temperatures of the intermetallic iron-zinc compounds (6 and 7 phases) may be exceeded without drawbacks. The zinc-containing a Fe crystalline solid solutions forming along with other phases will be bound by silicon in all cases in accordance with the known equilibrium relations in the Fe-Zn-Si system. In the drosses containing Fe and Si which are produced in the purification of zinc with silicon, these a-Fe crystalline solid solutions can often be demonstrated by testing for magnetizability.

Of advantage is also preheating or supplementary heating (for example, by radiation or induction) of the silicon added to the zinc melt. In thermal zinc production, it will suffice to deposit granular silicon or high-silicon alloys on the interface between liquid metal and zinc vapor.

EXAMPLE Through a charging valve, about to 250 g. of silicon metal containing 98.5% Si is introduced approximately every two hours into the spray condenser space of an electrothermic zinc reduction furnace. The amount of silicon is roughly equal to 200 g. Si per ton of metallic zinc produced. Through this addition, the average iron content of the zinc obtained without silicion treatment is reduced from 0.015 to 0.0025 Fe. The reaction products of this refining operation (FeSi for example) are found together with excess silicon metal in the zinc dust and in the dross.

What is claimed is:

1. Method for removing metallic impurities selected from the group of iron, cobalt, nickel, manganese or mixtures thereof from molten zinc which comprises admixing molten zinc with silicon thereby binding said impurities and thereafter separating the thus bound impurities.

2. Method of claim 1 wherein the silicon is deposited in granular form on the liquid zinc/zinc vapor interface.

3. Method of claim 1 wherein the silicon is dipped into the impure liquid zinc and good contact between the zinc melt and the silicon is promoted by mechanical or inductive stirring.

4. Process of claim 3 wherein the silicon is preheated prior to being dipped into the liquid zinc.

5. Method of claim 1 wherein the silicon is contacted with the liquid zinc by blowing, stirring, whirling or shaking the Si into the zinc melt.

6. Method of claim 1 wherein preheated silicon is poured over liquid zinc which is overheated in the presence of a reducing atmosphere.

7. Method of claim 1 wherein preheated silicon is deposited in granular form on an agitated zinc melt in the presence of a nonoxidizing atmosphere.

an alloy with a high silicon content.

4 References Cited UNITED STATES PATENTS 905,280 12/1908 Betts 7586 X 2,029,898 2/1936 Schmidt et al. 75-63 X OTHER REFERENCES J. W. Mellor: A Comprehensive Treatise on Inorganic and Theoretical Chemistry, vol. VI, Longmans, Green & Co., N.Y. 1925, p. 168.

OSCAR R. VERTIZ, Primary Examiner G. ALVARO, Assistant Examiner US. Cl. X.R. 

